Failed to open a session for the virtual machine BitcrazeVM.
VD: Backend 'VMDK' does not support discard (VERR_VD_DISCARD_NOT_SUPPORTED).
Result Code: E_FAIL (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}

I had the same problem.
I downloaded the "Oracle_VM_VirtualBox_Extension_Pack-5.2.18.vbox-extpack"
from the virtualbox website.
Install it by double clicking on it (Should have virtualbox open, I think).
Virtualbox will install it.